The house is brick + block, with a cast concrete floor, 2 storeys and
has probably used its permitted development rights when the previous
owners
had the garage converted into a couple of habitable rooms and a further
bedroom with en-suite built above -

Check with planning. The garage conversion might not have used any PD rights
at all if it didn't increase built volume. Even if it involved extending the
roof, there may be some left.

Christian.